One third of the buildings in central Christchurch were so badly damaged in last week's earthquake they may need to be demolished, authorities in New Zealand have warned, as the death toll from the country's worst-ever disaster reaches 145, with 200 still missing.

The city's central business district will take several months to recover, earthquake recovery minister Gerry Brownlee said, adding that "most of the services, in fact all of the services that are offered in the CBD, will need to relocate elsewhere".

Damaged buildings will need to be bulldozed and rebuilt "so that people can have confidence about coming back into the area to transact any business that's here", he added. One in three of the central city's buildings were severely damaged in the quake and must be demolished, according to earthquake engineer Jason Ingham.

The stark warning came as New Zealand's prime minister, John Key, said the quake was the country's worst disaster to date. "This may be New Zealand's single-most tragic event," he said.

Key said the government would announce an aid package on Monday for an estimated 50,000 people who will be out of work for months due to the disaster. He also called for two minutes of silence on Tuesday to remember both the victims and the ordeal of the survivors.

Previously, New Zealand's worst disaster had been held to be the 1931 Napier earthquake on North Island, in which at least 256 people died.

Police say up to 120 bodies may be entombed in the ruins of the downtown CTV building alone, where dozens of foreign students from an international school were believed to have been trapped.

The King's Education language school released a list of missing people presumed in the building: nine teachers and 51 students 26 Japanese, 14 Chinese, six Filipinos, three Thais, one South Korean and one Czech. An additional 20 students were listed with "status unknown".

A British specialist rescue team arrived on Friday to help search for survivors among the flattened buildings. They have been working among the ruins of the Pyne Gould Corporation building in the centre of the city.

Meanwhile, British families waiting for news of their loved ones caught up in the disaster were clinging to hopes for their safety.

Jo Morley, 44, whose brother Phil Coppeard from Bury St Edmunds in Suffolk is among the Britons missing, was taking comfort from the miraculous tales of survival following the Haiti earthquake. She said: "Of course you still hope, as anybody would do. That's what we have to think."

Coppeard, 41, a chartered accountant, emigrated to the country in November with his wife Suzanne Craig, and was doing a masters in economics at the University of Canterbury. He was travelling into town on a bus when the earthquake struck.

Christchurch Police Superintendent Russell Gibson said rescuers were not completely ruling out good news. "I talked to experts who say we've worked on buildings like this overseas and we get miracles. New Zealand deserves a few miracles," he said.

But families have also been warned to prepare for the "worst type of news", with New Zealand authorities admitting they expected the death toll "to steadily rise" over the weekend.

Two Britons have been confirmed to be among the dead, but that number is expected to grow as the process of identifying the bodies speeds up.

British victim, chef Gregory Tobin, 25, from Tadcaster, North Yorkshire, had been on a round-the-world trip and was believed to have been working temporarily at a garage in Christchurch when disaster struck. The identity of the other British victim, also male, has not been confirmed.

A multinational team of more than 600 rescuers from New Zealand, the UK, the US, China, Taiwan, Japan, Mexico and Australia continued scouring the city today but recovered only bodies.

Their efforts were hampered by a number of fresh aftershocks which sent masonry from a number of unstable buildings tumbling. So far only six quake victims have been formally named.